Using intracellular microelectrode techniques, we can determine the steady state resistances of the apical and basolateral membranes and the paracellular shunt pathway in toad urinary bladder. In addition, it is possible to determine the electromotive force (the potential difference which would exist in a circuit were it the only element present) at each border of the cells. Using this technique, we have been studying the permselectivity of the outer membrane of the epithelial cells for this tissue, and the effects of changes in solution on those permselectivity properties. Results have indicated that the outer membrane is indeed largely selective for sodium, but also has easily measurable anion permeability and selectivity. In addition, the sodium permeability is affected by changes in the anion concentrations in the mucosal bathing medium. Finally, changes in the serosal solution also effect the permselectivity of the apical (that is, mucosal or outward facing) barrier, indicating a form of correspondence between the two opposite membranes of the cells. In addition, we are passing rapid sequential pulses of varying magnitude across the entire tissue as a means of studying the current-voltage relation of the tissue. The data clearly indicate that changes in the current-voltage relation are due to changes in the shunt conductance (that is, the conductance of the paracellular pathway) as a function of voltage. We are also doing similar current-voltage studies on the apical membrane with the use of an intracellular microelectrode. We have been able to show that there is a relationship between the shunt conductance and the apical membrane conductance, and that both of these are in some ways related to the activity of the basolateral sodium pump in the toad urinary bladder.